1. Field of the Invention
The present invention relates to an optical transmitter with an auto-power control (hereafter denoted as APC) circuit able to detect a time differential of a bias current applied to a semiconductor laser diode (LD).
2. Related Prior Art
Conventional optical transmitter generally installs the APC circuit to stabilize an average power and an extinction ratio of an optical output from the transmitter. A photodiode (PD) installed within the optical transmitter detects a portion of the output light from the LD, and a controller that is also installed within the transmitter adjusts bias and modulation currents provided to the LD so as to keep the detected output light constant.
To detect an extraordinary status of the APC circuit, various techniques have been well known such as methods disclosed in Japanese Patent published as JP-2000-022631A or in the U.S. Pat. No. 6,952,531. The former prior art has disclosed the APC circuit that, when the bias current to the LD abnormally increases due to the degradation of the LD, sets an alarm of the bias current and changes the driving mode of the LD to generate an optical power with a predetermined strength. The latter art has disclosed another APC circuit configured to compare signals each input from the transmitter and the receiver with a reference and to turn off the transceiver according to the comparison.
Various factors to cause an abnormally status in the APC circuit are: (1) excess power from the LD due to a failure in the monitoring circuit including the PD, and (2) insufficient power of the LD due to the aging of the LD. These failures are detectable by the controller as an extraordinary in the bias current. For the former case, it is preferable to turn off the LD promptly from the view point of the eye safety or to prevent the optical receiver from being damaged by the excess powered light. On the other hand, for the second case, it is preferable only to set an alarm flag for the upper system but to continue the optical output to maintain the optical communication because, even the output power degrades, the optical communication may be often carried out.
However, the conventional APC circuit sometimes fails to maintain the optical transmission because, when the LD is replaced due to the degradation of the optical output by the aging, a fixed reference can not be adapted to the new LD. This is due to a reason that above two cases are undistinguishable to each other by the conventional system when the alarm is detected at the replacement of the LD. While, the method for shutting down the optical output based on the comparison of the optical output with the reference sometimes causes the failure of the whole communication system because of the complete shutting down of the optical signal.
The present invention has a feature to provide an optical transmitter that enables, in the APC circuit, to control the optical output depending on reasons to set the alarm.